It is becoming more and more common to use medicament delivery devices of the injection type wherein the injection sequence in many instances is performed automatically upon activation by the user. In the majority of the medicament delivery devices the injection is performed by a plunger rod acting on a stopper inside a medicament delivery device, where the plunger rod is urged forward by a force means, generally some sort of spring such as a compression spring.
Although the use of injection springs works well in many instances, they have a few drawbacks. One is that they tend to make the device rather long because it has to house the length of the plunger as well as at least a part of the injection spring, even though a part of the spring may be positioned inside the plunger rod. Another drawback is how to handle the force from the injection spring. In some devices, the device is delivered from factory with a pre-tensioned spring. Because there is a risk that the device may be stored for long periods of time before use, the design has to take care of problems with material creeping due to the built-in force, whereby certain components have to be designed with a higher safety factor and/or other types of materials, e.g. metal instead of plastic, which makes the device more expensive. In other devices the device is delivered without the spring being pre-tensioned, and the device is designed such that the tensioning of the spring is performed by the user prior to injection. The drawback here is that the device requires additional components in order for the user to perform the tensioning of the spring.
A few attempts with devices without injection springs have been done. The document WO 00035520 discloses an injector having a reservoir containing compressed gas. When a user presses an activating button a seal is ruptured in the reservoir wherein the compressed gas is led to act on a floating plunger. The plunger in its turn pushes against mated pistons thereby expelling a drug. The drawback with this solution is that it requires a component, the gas reservoir, which is at least as expensive as a spring, which requires quite a lot of space in the device, and also needs to be replaced if the injector is to be used several times. Further, the long time storage may affect the reservoir in many negative ways such that the device may not be functional anymore.
Another aspect of the invention is that it is becoming more and more common to use multi-chamber medicament cartridges in medicament delivery devices such as injectors. The reason for this is that the medicament can be stored for much longer time periods without being degraded in comparison with medicament dissolved in some liquid.
Thus the medicament and the liquid are kept in different compartments in the medicament cartridge and are mixed just before use by moving a dividing wall or stopper such that the compartments can communicate with each other.
However, the multi-chamber medicament cartridges entail more handling steps before a dose of medicament can be injected in that the plunger rod of the injector has to move the stopper of the medicament cartridge in order to initiate the mixing.
A number of solutions have been proposed for obtaining the mixing, from manual operation such as bringing together two parts of the injector to an automatic operation. A manual operation is described in EP 0 288 443 in which a front cover of the injector is rotated whereby a medicament powder chamber is pushed against a plunger, which in turn breaks an aluminium membrane such that liquid is mixed with the powder. This design is rather simple and requires few components, i.e. a robust design and function is obtained.